Cocoon thread discharged from a silkworm is constituted by two kinds of proteins which are a highly crystalline fibroin and a non-crystalline sericin. The sericin is present in a conglutinated state so as to surround two fibroins. A fiber after removal of sericin by scouring and containing fibroin as a main component is what is called silk thread. Heretofore, sericin has been discarded to waste as being of no value.
Recently, however, it has turned out that sericin possesses such properties as moisture retaining property, anti-oxidation action, cell protecting action, and protein protecting action, and is superior in biocompatibility. With this finding, attempts to utilize sericin as such a functional material as medical or cosmetic material have come to be conducted actively.
For example, in JP 3(1991)-284337A there is described a crosslinked polymer separation membrane formed by crosslinking sericin with formaldehyde and a heat-reactive water-soluble urethane resin into a thin membrane. In JP 6 (1994)-80741A there is described a protein-containing synthetic polymer material formed by emulsion-polymerizing a protein such as sericin with acrylonitrile and a protein-containing synthetic polymer material formed by joining and crosslinking a protein such as sericin with a water-soluble epoxy compound and a crosslinking agent into a three-dimensional network structure. In JP 2001-106794A there is described a polymeric hydrous gel formed by crosslinking a blend of sericin and polyvinyl alcohol by a crosslinking agent into an insoluble state. In JP 2002-201363A there is described a composite resin consisting of sericin and a polyvinyl alcohol-based water-soluble resin and capable of being subjected to thermoforming in a melted state.
However, in all of the above conventional techniques, sericin is made difficult to dissolve or insoluble with use of a synthetic polymer or the properties of sericin are slightly imparted to a synthetic polymer. Safety against environment and living bodies is not satisfactory and a limit is encountered in using the respective products as functional materials, especially as medical materials.
On the other hand, for example in JP 11(1999)-228837A there is described a silk protein/collagen composite prepared by evaporating to dryness a mixed aqueous solution or dispersion of silk protein and collagen and allowing solidification to take place. In JP 2003-192807A is described a silk protein cast film formed by drying an aqueous solution of silk protein in an inert atmosphere condition. It is described therein that the silk protein is either sericin or fibroin.
Indeed these techniques appear to overcome the foregoing problem related to safety, but as to JP 11(1999)-228837A, many of collagens available on the market are derived from cowhide and there is the recent tendency to refraining the use thereof because problems, including the problem of bovine spongiform encephalopathy, are coming into question.
Further, in JP 11(1999)-228837A and JP 2003-192807A, a silk protein is substantially fibroin alone or a mixture of fibroin and sericin, and even if sericin alone is used as a silk protein, it has so far been impossible to obtain a composite or cast film having a strength high enough to withstand practical use.
Subjecting sericin to freeze-drying is also performed for the purpose of preparing a powder of sericin. However, there has not been conducted yet any attempt to use sericin alone as a skeleton constituent of a porous body and obtain a porous body having a structural stability high enough to withstand practical use.